Wireless telephones, such as mobile/cellular telephones, cordless telephones, and other consumer audio devices, such as mp3 players, are in widespread use. Performance of such devices with respect to intelligibility can be improved by providing noise canceling using a microphone to measure ambient acoustic events and then using signal processing to insert an anti-noise signal into the output of the device to cancel the ambient acoustic events. Because the acoustic environment around personal audio devices such as wireless telephones can change dramatically, depending on the sources of noise that are present and the position of the device itself, it is desirable to adapt the noise canceling to take into account such environmental changes.
A typical adaptive noise cancellation (ANC) system may include a reference microphone for providing a reference microphone signal indicative of ambient audio sounds proximate to a personal audio device and an error microphone in proximity to a transducer for providing an error microphone signal indicative of the acoustic output of the transducer and the ambient audio sounds at the transducer. The typical ANC system may further include an adaptive feedforward filter that generates an anti-noise signal from the reference microphone signal to counter the effects of ambient audio sounds at an acoustic output of the transducer by adapting a response of an adaptive filter that filters an output of the reference microphone to minimize the ambient audio sounds in the error microphone signal based on a playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and a secondary path estimate. In addition, the typical ANC system may include an adaptive secondary path estimate filter for modeling an electro-acoustic path of the source audio signal that generates a secondary path estimate from a source audio signal by adapting the response of the secondary path estimate adaptive filter to minimize the playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the secondary path estimate. The typical ANC system may combine the anti-noise signal with the source audio signal to generate an audio signal provided to the transducer.
Such an ANC system requires the source audio signal in order to properly adapt or “train” the response of the secondary path estimate filter. However, a disadvantage of training with a source audio signal may be that such signals may not have the persistence or spectral density required to effectively train the response of the secondary path estimate filter, in that a source audio signal may have silent intervals or may lack content in particular ranges of frequencies. Such disadvantage may particularly be present in stereo playback modes, as each channel of the stereo signal may convey only a portion of the source audio signal.